Currently, LiCoO2 is a conventional cathode material for high energy density lithium ion batteries. Under the limit of the compact density and the usage upper limit voltage, its energy density has reaches its maximum value and the battery is highly polarized under that energy density, so it's difficult to meet the requirements of high output power and long service life. Compared with LiCoO2, layered lithium-nickel-cobalt-aluminum oxide cathode material LiNi1-x-yCoxAlyO2 (wherein 0<x≦0.2, 0<y≦0.1), because of its advantages on high specific discharge capacity (180˜230 mAh/g) and low cost, are drawing increasingly attention in the field of lithium battery. It is expected to be one of cathode materials with the best market prospect in the future. However, the material itself still has some drawbacks to be solved:                (1) Poor processability. Due to difficulties in the oxidation of Ni2+ to Ni3+, generation of Ni2+ is inevitable during the preparation process of LiNi1-x-yCoxAlyO2. In presence of Ni2+ in LiNi1-x-yCoxAlyO2, some Ni2+ takes the positions of Ni3+, which consequently decreases cationic charge. To maintain the charge balance, some Ni2+ take the positions of Li+, leading to free Li+ increase and reactive oxygen deintercalation inside structure. The reactive oxygen further react with Co2 and H2O in the air to generate Co32− and OH−. The generated Co32− and OH− continue to react with free active Li+ to form impurities Li2Co3 and LiOH. Even at room temperature, there are Li+ ions are deintercalated on the surface of the material to form Li2Co3. Formation lithium impurities (Li2Co3 and LiOH) result in elevated pH value and proportionally increased water content on the surface of the material, which lead to the poor dispersity and stability of the slurry. Thereby the preparation process of the slurry is difficult because it is easy to gel.        (2) Poor storage and cycling performance at high-temperature. On one hand, the electrolyte LiPF6 can easily react with the water, which is brought in by the cathode material LiNi1-x-yCoxAlyO2, to generate HF during storage and cycling at high temperature. HF further reacts with Li2Co3 and LiOH impurities to produce Co2 and H2O which serve as an initiator and further catalyze the decomposition of LiPF6. On the other hand, during the charging process of LiNi1-x-yCoxAlyO2, Ni2+ and Ni3+ will gradually be transformed into oxidative and active Ni4+, along with the deintercalation of the lithium ions. During the storage and cycling process at high temperature, the Ni4+ can easily catalyze the decomposition of the solvent in the electrolyte system, which result in generation of a lot of Co2 and influences the performance of the lithium ion battery with LiNi1-x-yCoxAlyO2 as the cathode material. For lithium ion battery with an aluminum-plastic composite film as outer package, because the outer package is soft, the generated gas will result in swelling and deformation of the lithium ion battery and bring severe safety hazard.        
The Chinese patent CN102496710A discloses a nickel based multi-component cathode material coated with lithium phosphate and metal oxides and a method of fabricating the same. This invention utilizes the barrier effect of lithium phosphate and metal oxides and solves the problem of decomposition of the solvent in the electrolyte system by highly oxidative and active Ni4+ at high temperature. However, as the coating of lithium phosphate and metal oxides are fabricated separately, it does not achieve the desired effect of reducing the amount of free lithium impurities on the surface of the material, which results in poor processability of the material. Meanwhile, due to presence of too much lithium impurities, the improvement on the thermal stability of the battery is limited.
The Chinese patent CN103337614A discloses a modification method of nickel based metal oxide cathode material LiNixM1-xO2, wherein the cathode material is washed with an alcohol and organic acid-mixed solution. This modification method removes the soluble impurities of the lithium salts on cathode material, significantly decreases the pH value of the material and partly improves the high-temperature storage and cycling performance of the battery. However, this method cannot solve the catalytic decomposition of the solvent in the electrolyte system by highly oxidative and active Ni4+, and thus there still is comparatively much Co2 production upon high-temperature storage and high-temperature cycling, which limits the performance of the battery and cannot meet the demands of current electric vehicles on power battery.